Referring to FIG. 1, positive temperature coefficient (PTC) over-current protection device exhibits a PTC effect that renders the same to be useful as a circuit protection device 1. A conventional circuit protection device 1 includes a PTC polymer layer 11 and two electrodes 12 attached to two opposite surfaces of the PTC polymer layer 11. The PTC polymer layer 11 includes a polymer matrix that contains a crystalline region and a non-crystalline region, and a particulate conductive filler dispersed in the non-crystalline region of the polymer matrix and is formed into a continuous conductive path for electrical conduction between the electrodes 12. The PTC effect is a phenomenon that, when the temperature of the polymer matrix is raised to its melting point, crystals in the crystalline region start melting and results in generation of a new non-crystalline region. As the new non-crystalline region is increased to a certain extent and merges with the original non-crystalline region, the conductive path of the particulate conductive filler will become discontinuous and resistance of the PTC polymer material will increase rapidly, thereby resulting in electrical disconnection between the electrodes.
The circuit protection device is used for protecting an electronic apparatus, and the polymer matrix of the PTC polymer layer 11 is selected based on an operating current and an operating voltage of the electronic apparatus. The polymer matrix of the PTC polymer layer 11 is usually made from a polyethylene-based composition. However, the circuit protection device might not have desired electrical conductivity due to a relatively poor adhesion between the PTC polymer layer 11 and the electrodes 12.
U.S. Pat. No. 6,238,598 discloses a PTC polymer blend composition and a circuit protection device. The PTC polymer blend composition includes a non-grafted polyolefin, a grafted polyolefin, and a conductive particulate material. The circuit protection device includes a PTC element having the PTC polymer blend composition and two electrodes respectively connected to two opposite sides of the PTC element. With inclusion of the grafted polyolefin in the PTC polymer blend composition, the circuit protection device has a relatively good electrical stability and good adhesion between the PTC element and the electrodes.
However, volume resistance of the circuit protection device disclosed in U.S. Pat. No. 6,238,598 is not lower than 0.15 ohm-cm. The volume resistance may be reduced (e.g., close to 0.1 ohm-cm) by virtue of excluding the grafted polyolefin from the PTC polymer blend composition. However, peel strength between the electrodes and the PTC element may become lower than 0.4 kG/cm2. As a consequence, the PTC element and the electrodes may be easily separated from each other, thereby resulting in malfunction of the over-current protection device.
In view of the foregoing, there is a need for manufacturing an over-current protection device with desired conductivity and good adhesion.